测试文档锁：doc.LockDocument()

/// <summary>
        /// 总结：用到DocumentManager.Open(filePath)时，如果是ForWrite，就需要用到lock文档锁。
        /// </summary>
        [CommandMethod("T38")]  //测试文档锁
        public void T38()
        {
            //Document doc = Application.DocumentManager.MdiActiveDocument;  //如果是MdiActiveDocument，不需要锁LockDocument。
            Document doc = MyTool.MyTools.OpenDwgFile(@"E:\res\autocad\test.dwg");
            Database db = doc.Database;
            //Database db = new Database();
            //db.ReadDwgFile(@"E:\res\autocad\test.dwg", FileShare.Read, true, null);  //这种方式打开db，不要要锁。
            using(doc.LockDocument())   //这里不锁的话，ForWrite模式打开就会报错，但ForRead模式不报错。但如果是MdiActiveDocument，也不需要锁。
            using (Transaction trans = db.TransactionManager.StartTransaction())
            {
                BlockTable bt = db.BlockTableId.GetObject(OpenMode.ForRead) as BlockTable;
                BlockTableRecord modelSpace = bt[BlockTableRecord.ModelSpace].GetObject(OpenMode.ForWrite) as BlockTableRecord;
                /*
                ObjectId bRefId = ObjectId.Null;
                foreach (ObjectId id in modelSpace)
                {
                    if (id.GetObject(OpenMode.ForRead) is BlockReference)
                    {
                        bRefId = id;
                    }
                    break;
                }
                BlockReference bRef = bRefId.GetObject(OpenMode.ForWrite) as BlockReference;
                bRef.DowngradeOpen();
                 * */
            }
        }